Contreras is a former pitcher for the Cincinnati Reds, New York Mets, Philadelphia Phillies and the Chicago White Sox organizations before getting an opportunity for pitching in the major leagues for the White Sox in 1980. He finished his playing career with the White Sox AAA team in 1981 and 1982. Following his playing career, Contreras has served as a coach for several organizations at the major league level including the New York Yankees in 1995, Seattle Mariners in 1997 and 1998, and for the Chicago White Sox from 1998-2002, all as their major league pitching coach. In 2005 he returned to the Yankees organization as their minor league pitching coordinator.